<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
login_fail<br>
model,看不到:<span th:text="${username}"></span><br>
redirect_msg:<span th:text="${param.redirect_msg}"></span><br>
只显示一次：<span th:text="${flash_msg}"></span>
</body>
</html>